The present invention relates to a video cassette recorder, in particular, it relates to a video cassette recorder having SP/LP modes and SD/SDL modes.
In recent years, digital video cassette recorders for public use are spreading. Digital video cassette recorders for public use have better picture quality and are able to record longer programs in comparison with conventional analog system VTR. It is desired that a digital video cassette recorder can be operated in the same way as a conventional analog video cassette recorder is operated, and in particular it is needed that in the cases of a normal mode (SD mode) and a long play mode having a xc2xd tape speed of the normal tape speed, a digital video cassette recorder can be operated in the same way as an analog video cassette is operated. And also, it is desirable that the SP mode and the LP mode which have different track pitches can be used in the same way.
Accordingly, there are four independent modes in the format of a digital video cassette recorder for public use which are brought about from respective combinations of SP/LP modes having different track pitches and SD/SDL modes having different compression rates.
Among these, the track pitch in the SP mode is 10 xcexcm. In the LP mode where the tape speed is 0.667 time of that in SP mode, the track pitch becomes 6.67 xcexcm.
In the case of SDL mode, the recording information quantity per unit time is made half because of high compression rate and the tape speed is also made half of the tape speed in SD mode due to an intermittent recording which is performed once in two rotations.
Head constitutions for realizing SDL are described in the following:
1. There is a constitution in which a double gap head is used. However, in this method, it is technically difficult to make uniform the contact between the heads and the tape.
2. There is another constitution in which two pieces of +azimuth heads for SDL are separately used for the SD mode and for the LP mode. However, the constitution becomes four heads constitution including the +azimuth heads for SD mode, which makes it difficult to make the contact between respective heads and a tape uniform. With the increase in the number of heads, the head beating sound becomes larger, and the number of rotary transformer is also increased, which makes the structure of a drum complicated. The time for adjustment work is also increased with the increase in the number of heads because it is needed to adjust playback equalization characteristics about every head.
There is a problem that even in the case of the same track width, the output on the xe2x88x92azimuth side becomes smaller than that on the +azimuth side by about 1 dB caused by the relations xc2x1azimuth angles between the tape orientation and the REC angle. Further, there is another problem that the side erase due to the exudation of magnetic field of a head upon recording gives more influence to the xe2x88x92azimuth side than to the +azimuth side and the effective residual width on the xe2x88x92azimuth side becomes narrower than that on the +azimuth side by about 1 xcexcm.
In particular, since the LP mode is a narrow-track-pitch-mode, error rate margin till the critical error rate, that is, the time interval till the time when the output of the head goes into concealment is very small. Therefore, the output on the xe2x88x92azimuth side is decreased and the possibility of breaking the critical error rate becomes high with a small degradation of output.
As mentioned in the above, in a conventional digital video cassette recorder for public use, a plurality of heads are needed to realize a long play mode; thereby, there has been a problem that the constitution of a drum becomes complicated, that is, the cost of the recorder becomes high. There has been another problem that the difference in output levels among channels with azimuth angles is produced.
The object of the present invention is to realize a digital video cassette recorder in which a long play mode can be realized with a comparatively simple method, with a number of heads as small as possible, and also with no imbalance in the outputs among channels.
In order to achieve the above-mentioned objects, a digital video cassette recorder according to the present invention is constituted as described in the following: the digital video cassette recorder having a plurality of heads provided on a rotary member, and performing helical scanning for a magnetic recording medium with these heads, and performing recording/playback of data on slanted tracks in which two adjacent tracks have different azimuth angles from each other, wherein the heads being composed of a first head and a second head having different azimuth angles from each other being disposed opposing to each other on the rotary member at an angle of 180xc2x0, and a third head disposed at an angle of 270xc2x0 from the first head in the rotation direction and having the same azimuth angle as that of the second head, and comprising a head changeover means for changing over the first, the second and the third heads, and the digital cassette recorder comprising two modes having different compression rate, wherein in a first mode, recording/playback being performed switching the first and the second heads alternately, in a second mode, the tape speed being lowered to a half and recording/playback being performed once in two rotations switching the first and the third heads alternately.
The digital video cassette recorder having a plurality of heads provided on a rotary member and performing recording/play-back of data on the slanted tracks in which two adjacent tracks having different azimuth angles in performing helical scanning for a magnetic recording medium with these heads, wherein fixed heights of heads are made to differ in every head, in order to dissolve the difference in the outputs caused by the difference in azimuth angles related with the direction of orientation of a magnetic recording medium and the recorded direction of the slanted tracks, and also to dissolve the difference in the out-puts caused by the imbalance of side erase.